Doktorsavhandling, 2004

The thesis presents the development of microwave tunable devices based on
ferroelectric thin films. The main tasks in this work are the fabrication, optimization, and
integration of ferroelectric thin film into Si-MMIC. This work has been motivated by
increased interest on advanced tunable microwave components which meet the
requirements of low microwave losses, high tunability, easy to integrate to larger
systems, and low cost.
Dielectric permittivity, tunability, and loss tangent are the most important parameters,
characterizing ferroelectrics for application in tunable microwave devices. Most of the
efforts in this work have been devoted to the optimization of film fabrication processes in
terms of microwave loss reduction. Experimental investigations of these parameters for
Na0.5K0.5NbO3 (NKN), (SrTiO3) STO, Ba0.25Sr0.75TiO3 (BSTO) and (Pb, Zr) TiO3 (PZT)
films are carried out in a wide frequency, DC bias, and temperature ranges. These
measurements give important information about dispersion, temperature, and dc-field
dependencies of dielectric permittivity and losses. The latter are useful for understanding
the basic physical processes in ferroelectric films and help to optimize film deposition
and device fabrication processes in terms of low losses, high tunability, and temperature
stability.
Coplanar-plate structure is used to integrate the ferroelectric films into the silicon
circuits. Planar varactors are fabricated by depositing tunable ferroelectric films on
Si/SiO2 substrates using laser ablation, RF magnetron sputtering, and sol-gel technique,
followed by metal electrode deposition. Different varactor designs based on as straight
gap, interdigital gap, and circular gap have been investigated and further used in the
components. Experiments show the capability and potential of tunable devices based on
ferroelectric thin films and confirm that they are competitive with other technologies like
semiconductors at frequency above 20 GHz. It is shown that NKN films in polar phase
are also useful for tunable microwave applications demonstrating low loss and good
tunability in a frequency above 20 GHz. BSTO films, which are in paraelectric phase at
room temperature, are used to fabricate microwave varactors. These varactors are used in
two different phase shifters designs. These tunable devices show more than 60°/dB figure
of merit, which is one the best value reported so far.